A movable iron core linear actuators, such as a reciprocal motor, includes, as a main component, a magnetic circuit in which a moving element reciprocates when being energized as illustrated in, for example, Patent Document 1. The magnetic circuit includes an iron core constituting a moving element, a stator core including a facing portion which faces the iron core, a pair of permanent magnets disposed in a facing portion along the reciprocating direction and having inverted magnetic poles at their surfaces which face the iron core, and a coil wound around the stator core. A magnetic flux produced by energization of coil weakens a magnetic flux produced in a magnet situated in a necessary direction between the pair of permanent magnets, and strengthens a magnetic flux produced in the other magnet. Therefore, the moving element is made to reciprocate with respect to the stator core.
The magnetic circuit causes self holding force to act on the moving element by the magnetic flux produced by the permanent magnets in a state in which the coil is not energized. The self holding force is not external force which acts on the moving element by a mechanical element, such as a flat spring, but self force produced in the moving element by the magnetic flux of the permanent magnets. The self holding force acts on the moving element so as to return the moving element to a predetermined position if the moving element has been displaced from the predetermined position. This self holding force is also called offset force produced by the magnetic flux of the permanent magnets.
If, however, the position of a linear actuator in which offset force by the permanent magnets is set so as to return the moving element to the center of the movable range of the moving element in a state in which the linear actuator is at a horizontal position is changed to a longitudinal position, the offset force by the permanent magnets becomes smaller than the gravity which acts on the moving element at the center of the movable range and the moving element is displaced downward from the center of the movable range. In this case, there is a problem that, in order to avoid collision with, for example, an unillustrated casing which houses the moving element and the stator, a reciprocatable range of the moving element becomes narrower than original movable range, and therefore it is not possible to cause the moving element to reciprocate with an amplitude that requires a range wider than the range in which reciprocation is possible.
As a means to solve this problem, Patent Documents 1 and 2 disclose a device which adjusts the offset position, which is a balanced point of the gravity acting on a moving element and the offset force by the permanent magnets, is situated at the center of the movable range. In the device, positions of the permanent magnets are determined or strength of each permanent magnet is made to vary such that the magnetic flux of the permanent magnets is biased when not energized, and therefore the offset force by the permanent magnets is increased upward.